1. Field of the Invention
The present invention relates to a polishing method and a polishing apparatus and, more particularly, to a polishing method and a polishing apparatus which polish a work surface still having an unevenness thereon with slurry-containing particles.
2. Description of the Related Art
As disclosed in Japanese Patent Application Laid-Open No. 11-288906, a CMP (Chemical Mechanical Polishing) method has widely been used conventionally in the process of planarizing a semiconductor wafer substrate.
Especially, an enhanced integration degree of VLSI circuits causes an increase in number of wiring layers, and improving the planarization process with a CMP method is strongly required. The planarization process can be possible by realizing selective polishing on the protrusions of a surface. Conventionally, a polishing pad is pressed to the surface at constant pressure so as to increase the speed at which the protrusions are selectively polished and removed with the aid of the pressure-distribution difference between the protrusions and depressions. However, since the method cannot produce a sufficient difference in the pressure distribution, there is a limitation on the ability to reduce unevenness. Thus, a method which actively allows a material process in an arbitrary fine region has been needed.
As shown in FIG. 1, the conventional CMP method involves adhering and fixing an elastic polishing pad 12 onto a rotating polishing plate 11. A silicon wafer 13 is fixed to an end face of a polishing head 14. The silicon wafer 13 is fixed by compression to the polishing pad 12, with its surface to be polished facing downward. Slurry 15 is supplied in this state, the polishing plate 11 is rotated and the polishing head 14 is also rotated to polish the surface of the silicon wafer 13.
At this time, since the polishing pad 12 is in contact with the silicon wafer 13 with pressure exerted on each other, the slurry 15 is not sufficiently flowed into the polishing region and results in unstable polishing. To prevent this, the surface of the polishing pad 12 is dressed with a diamond tool or the like so as to make relatively large protrusions and depressions for slurry pools. Thus, because the surface of the polishing pad 12 is made of an elastic material, it becomes fuzzy due to the protrusions and depressions for the slurry pools and scratching with the dressing tool.
As shown in FIG. 2, the silicon wafer 13 to be polished with the CMP method shown in FIG. 1 has regular protrusions and depressions of conductive metal at a surface layer, such as wiring pattern 21, and a thin film layer 22 serving as an insulating film made of SiO2 formed over the protrusions and depressions. Affected by the protrusions and depressions of the wiring pattern 21, protrusions and depressions are generated at the surface of the thin film layer 22. In the planarizing process with the CMP method, selectively polishing on only the protrusions 23 of the protrusions and depressions of the thin film layer 22 achieves a surface planarization.
Attempts have been made to bring the polishing pad 12 into contact only with the protrusions on the surface of the silicon wafer 13 to perform polishing by increasing the elasticity of the polishing pad 12, by way of example. Actually, however, as shown in FIG. 3, because the polishing pad 12 is formed of an elastic body which is deformed under pressure and its surface is fuzzy, the surface of the polishing pad 12 contacts not only the protrusions 23 of the thin film layer 22 but also the depressions. In other words, it is difficult to perform selectively polishing on only the protrusions 23.
For this reason, it is difficult to achieve an ideal planarization process which largely removes material including only the protrusions 23, as shown by a removed region 24 in FIG. 4, and selectively removes the protrusions 23. Specifically, in reality, there exists a problem that the removed region 24 has a substantially uniform thickness regardless of the presence of the protrusions and depressions, as shown in FIG. 5, to substantially and uniformly polish the protrusions and depressions of the thin film layer 22 made of SiO2 formed on the surface of the silicon wafer 13 even with the progression of the polishing, thereby preventing successful planarization.
When planarizing a surface having protrusions and depressions thereon, it is necessary to selectively remove only the protrusions. In a typical CMP, however, since the surface of the polishing pad has large protrusions and depressions for holding slurry and is dressed to have fuzz as fibers, the polishing pad is not in contact uniformly with the silicon wafer. When the protrusions are removed with the progression of the polishing, the depressions are also simultaneously removed, although the removed amount is smaller. For this reason, planarization in a short time is difficult in polishing the surface having the protrusions and depressions. Also, even when polishing the surface is performed in a long time, a favorable planarized surface is difficult to achieve although its steps are reduced.
This phenomenon is also seen in the processing of an aspheric lens. Specifically, in general, a high precision grinding process is performed to form a desired aspheric shape on the surface of a lens, and then a polishing process is performed to remove a surface damage layer and ensure surface roughness as an optics device simultaneously.
In the polishing process, even when measurements are made previously to calculate polishing regions and removing amounts at those regions, the shape of the region to be removed through the polishing has a certain area and thus its surroundings are processed at the same time, meaning that regions other than the intended region are subjected to processing. As a result, the polishing process deteriorates the accuracy achieved through the grinding process.
In view of the above-described problems, the present inventors have proposed a method, as shown FIG. 6, in Japanese Patent Application Laid-Open No. 2000-289444. The method can be carried out local and selective polishing of the protrusions 23 of the thin film layer 22 formed on the silicon wafer 13 by selectively irradiating laser light to the protrusions 23, so as to form an aggregation trace of particles of the slurry there, and then performing polishing to break the aggregation trace of particles to result in a high concentration of the slurry in a local region.
According to the above-mentioned method, however, most of the polishing time is spent on polishing the aggregation trace, and an only extremely short time is spent on removing the material of the protrusions 23 immediately before the aggregation trace is eliminated. Thus, the method has the disadvantages of an insufficient ability of planarization and a low efficiency of planarization.